The Smith-Purcell effect describes light emission (also referred to as Smith-Purcell radiation) from collective excitation that is induced by a free electron when the free electron couples, through its near field, to the electromagnetic modes of a periodic structure. The wavelength of the Smith-Purcell radiation usually depends on the velocity of the electron and the geometry of the periodic structure. Therefore, the Smith-Purcell radiation can be used to construct light sources that can be wavelength tunable via the adjustment of the electron velocity. Conventional systems utilizing the Smith-Purcell effect usually use metallic periodic structure due to the image charge intuition available in metal.
However, output from existing light sources based on the Smith-Purcell effect is usually too weak to be used for realistic applications. This may be attributed to several reasons. First, it is usually challenging to address the phase mismatch between the electrons with the emitted photons because the speed of electrons can hardly reach the speed of light. Second, the intensity of the Smith-Purcell radiation usually benefits from a short distance between the electrons and the periodic structure, but precise alignment tends to be difficult especially on nanoscale. Third, spontaneous Smith-Purcell radiation contains a wide range of frequency components, and each component typically radiates into a different direction. It remains a challenge to generate Smith-Purcell radiation that is more monochromatic and more directional.